Project Summary/Abstract Traumatic brain injury (TBI) is a growing and under-recognized public health threat. The Centers for Disease Control and Injury Prevention estimate that 2.5 million Americans sustain a traumatic brain injury each year. In fact, TBI-related healthcare costs eclipse 80 billion dollars annually. There are currently no effective therapies for TBI and supportive care remains the mainstay of treatment. The impact of TBI is highlighted not only by its high mortality but also by the significant long-term neurologic impairment complications suffered by survivors. The immune response to TBI plays a fundamental role in the development and progression of this subsequent neurologic impairment and represents a complex interplay between infiltrating monocytic cells and the resident immune system of the injured brain?microglia. Despite this, reciprocal action between monocytes and microglia is poorly understood and the molecular mechanisms driving their interaction remain largely unknown. Preliminary work has generated head-shielded bone marrow chimeric mice allowing for the unambiguous differentiation between infiltrating monocytes and microglia after TBI. Using this model, we have shown that non-classical monocytes are essential for neutrophil recruitment into the injured brain after TBI and that their targeted depletion results in improved functional and anatomic outcomes after injury. Furthermore, this model has allowed for the sorting of isolated populations of microglia after TBI. Transcriptional profiling of these microglia has implicated longitudinal changes in microglial gene expression in the development of long-term neurodegenerative changes. Taken together, we that infiltrating monocytes shape the microglial response to injury altering gene expression, anatomic, and functional outcomes after TBI. To test this hypothesize hypothesis, we will determine whether microglia adopt a TBI-associated phenotype after TBI and whether infiltrating monocytes are required for their generation. Additionally, our Preliminary Data has identified progressively increased expression of genes involved in synaptic plasticity in the microglia of TBI mice. In particular, Striatal-enriched protein tyrosine phosphatase (STEP) was identified as a key protein in this process. STEP is important in several other neurocognitive disorders, but has not been investigated in TBI. We will determine whether STEP, and other regulators of synaptic plasticity, contribute to the development and degree of neurocognitive dysfunction after TBI with the use of knockout and transgenic mice. Lastly, we will obtain monocytes from traumatically brain-injured human patients to develop a humanized mouse model of TBI. Using this model, we will determine whether autonomous changes in monocytes from TBI patients direct microglia to adopt a TBI-associated phenotype as compared to monocytes from healthy controls. Collectively, the proposed studies will identify key molecular events and pathways that govern microglia and infiltrating monocyte interaction in TBI, thus raising the potential for transformative biologic discovery and therapeutic development in TBI patients.